Leadframes are commonly used in the mass production of light emitting devices (LEDs). The leadframe may be a sheet or strip of metal that is etched or punched to provide a pattern of conductors for coupling to a light emitting element and providing pads or other elements that facilitate the use of the light emitting device in products or assemblies.
FIG. 1A illustrates a section of a conventional leadframe 100. The leadframe 100 includes openings 120 that form a pattern 110 of conductive elements. The leadframe 100 may extend for several feet, to provide patterns for situating each of a plurality of light emitting elements. The leadframe 100 travels in an assembly-line, wherein the various processes, detailed below, are performed to create individual light emitting devices.
The section illustrated in FIG. 1A includes conductive patterns for creating two light emitting devices. Although illustrated as a narrow strip with one column of patterns, a wider sheet may be used to provide an array of patterns. Each pattern includes a site 130 for situating the light emitting element, and pads 140A, 140B that facilitate mounting the finished light emitting device on a printed circuit board, or other assembly. The pattern 110 also includes conductive segments 135A, 135B (collectively “135”) that serve to facilitate coupling these pads 140A, 140B to the light emitting element.
To operate properly, the pads 140A, 140B must not be connected to each other; however, to hold these pads 140A, 140B (and other elements) to the leadframe, a plurality of tiebars 115 are used to maintain mechanical integrity of the pattern during processing by connecting pads 140A, 140B to the leadframe 100. Typically, tie bars are severed in later stage of processing.
FIG. 1B illustrates the leadframe 100 after being ‘populated’ with light emitting elements 150 and protective structure 160. A light emitting element 150 is situated at the appropriate site (130 in FIG. 1A) on the pattern 110, and coupled to the conductive elements 135A, 135B. In this example, one of the contacts of the light emitting element 150 is situated on the lower surface of the light emitting element 150, and the other on the upper surface. Mounting/bonding the light emitting element 150 upon the conductor segment 135B effects a coupling of the contact on the lower surface of the light emitting element 150 to the pad 140B, and a bondwire 155 is used to couple the contact on the upper surface of the light emitting element 150 to the pad 140A, via conductor segment 135A.
Structure 160 is formed around the light emitting element 150 so as to encompass a significant portion of the conductor elements 135. In this manner, the structure 160 provides structural support for the light emitting element 150 and the conductor elements 135. When structure 160 provides this mechanical support, the tie bars 115 may be severed, as illustrated by the “X”s in FIG. 1C.
FIG. 1D illustrates a resultant ‘singulated’ light emitting device 190. This device 190 includes a light emitting element 150 encased in a protective structure 160. The light emitting element 150 is electrically coupled to pads 140A, 140B for providing external power to the light emitting element 150, via conductor elements 135A, 135B.
The metal elements that are external to the structure 160 may subsequently be formed to a desired shape. FIG. 1E illustrates a profile view of the light emitting device 190 after forming a ‘step’ in the conductor elements 135 so that the pads 140A, 140B extend lower than the lower surface of the structure 160.
The light emitting element 150 will generate heat during operation. At low power, a sufficient amount of this heat may be dissipated through the structure 160 and the pad 140B (minimal heat is transferred to pad 140A via the bonding wire 155). At higher power, an external heat sink, or other heat dissipating mechanism, may be required to prevent thermal damage to the light emitting element 150. Such an external heat dissipating mechanism adds material cost to the finished product, requires an extra manufacturing step, further adding to the cost, and often requires modification to the basic structure of the light emitting device to accommodate this heat dissipating mechanism. For example, if a heat sink were to be installed beneath the structure 160 of the light emitting device 190, the extent of the conductors 135 may need to be increased, to increase the height of the structure 160 above the pads 140 in order to accommodate the height of the heat sink. Additionally, depending upon the thickness of the structure 160 beneath the conductor 135B, the thermal transfer from the light emitting element 150 to the heat sink beneath the structure 160 may be inefficient.